During the production of silicon single crystals, a supersaturation of lattice vacancies occurs during the pulling of the single crystals from the melt and subsequent cooling. These lattice vacancies combine to form cavities that, depending on the pulling process properties of pulling speed and cooling rate, have a mean size of approx. 100 nm and a density distribution that is dependent on the radius. Even if only a relatively small proportion of the crystal is disrupted, these crystal structural defects can nevertheless have a considerable influence on various properties of the crystal.
Deviations of this type may, for example, be octahedral cavities, known as “crystal originated pits,” or COPs for short, which form in the crystal as a result of vacancy agglomerations. Some of these cavities are randomly located at the wafer surface following the sawing and polishing of the silicon single crystal.
In MOS structures, SiO2 is used as electrically insulating gate oxide. Therefore, the reliability of the gate oxide is crucial to the reliability of electronic devices. Grown-in cavities that are randomly intersected by the wafer surface after the single crystal has been sawn into individual wafers, may locally reduce the insulating properties of the gate oxide that has been applied to the entire wafer surface. The reason for this is that the oxide has thin points at the edges and/or comers located within the cavity. On account of these thin points, the breakdown voltage is locally reduced. During continuing operation of the chip, a gate oxide breakdown can occur predominantly at these thin points, since the field strength is higher at these points. A component that is located at this point can thereby stop functioning.
Defects of this type occur with a mean density of 10 cm−2 in the silicon structure and lead to an increase in the failure rate. Standard semiconductor chips, depending on their active gate oxide area, which is on average 1 mm2, are affected by this failure mechanism to the extent of approximately 10%. Therefore, it is necessary to use a base material with a lower defect density. An improvement of this type in the surface properties of a silicon wafer is possible, for example, by using a complex crystal pulling process (perfect silicon) and/or an aftertreatment (high-temperature anneal). Cavities of this type can be closed up, in such a way as to eliminate their effect, by epitaxial growth of a single-crystal silicon thin film. However, all these measures lead to undesirable cost increases and/or are dependent on the manufacturer of the base material.